Field of the Invention
The present invention relates to an active matrix substrate used for a display device and the like and relates to a manufacturing method of the active matrix substrate.
Description of the Background Art
An active matrix substrate which uses thin film transistors (TFT) as switching elements (hereinafter, referred to as a “TFT substrate”) is widely known as one used for an electro-optical device, for example, a display device which uses liquid crystal (hereinafter, referred to as a “liquid crystal display device”) and the like. With respect to electro-optical devices using TFT substrates, the display performance is required to be improved (a higher definition, a higher quality, and the like), and cost is required to be reduced by simplifying a production process and producing efficiently.
Conventionally, as switching elements in a TFT substrate for a liquid crystal display device, a back-channel type TFT is widely used. In the back-channel type TFT for a liquid crystal display device, amorphous silicon (Si) is used for a semiconductor active layer, and the back-channel type TFT is generally made in an element structure called a “reverse stagger type.” A TFT substrate equipped with reverse stagger type TFTs is usually manufactured through four to five photoengraving processes (photolithography processes).
Alternatively, from the point of view of simplifying a production process of a TFT substrate, a manufacturing method is disclosed in, for example, Japanese Patent Application Laid-Open No. S64-35529 (Japanese Patent Application Laid-Open No. H1-35529), No. 2001-056474, and No. 2004-281687 (Japanese Patent No. 4522660), in which manufacturing method TFTs having an element structure called a “stagger type” are used as switching elements, and three photolithography processes are used to form the TFT substrate.
On the other hand, from the viewpoint of display performance, a technology has been developed in which an oxide semiconductor having higher mobility than the conventional Si is used for an active layer of the TFT (for example, see Japanese Patent Application Laid-Open No. 2004-103957 (Japanese Patent No. 4164562) and No. 2005-77822 and Nature Vol. 432 (2004) p. 488). As an oxide semiconductor, research and development are conducted focusing around a zinc oxide (ZnO)-based oxide semiconductor and an IGZO-based oxide semiconductor in which gallium oxide (Ga2O3) and indium oxide (In2O3) are added to zinc oxide (ZnO).
The above-described oxide semiconductors can be etched with a weak acid solution such as oxalic acid and carboxylic acid, which provides an advantage to be easily patterned. However, the above-described oxide semiconductors are easily dissolved with acid-based solutions which are usually used in etching processes of common metal films (Cr, Ti, Mo, Ta, Al, Cu, and an alloy of them) used for source electrodes and drain electrodes of TFTs. For this reason, in the case of forming a TFT having an active layer of an oxide semiconductor, it is necessary that a new element is added to the oxide semiconductor to improve chemical resistance and that film thicknesses of the metal film and the oxide semiconductor are optimized such that the oxide semiconductor as the active layer does not disappear when the metal film as the source electrode and the drain electrode is etched (patterned) as described in, for example, Japanese Patent Application Laid-Open No. 2008-72011.
Since the FFS (Fringe Field Switching) type liquid crystal display panel disclosed in Japanese Patent Application Laid-Open No. 2001-056474 has excellent field-of-view characteristics and transmittance of the panel, demand for the panel is increasing. However, in the FFS type liquid crystal display panel, pixel electrodes and an opposite electrode (common electrode) for generating electric fields to drive liquid crystal are both required to be formed on the TFT substrate, whereby the number of the wiring layers on the TFT substrate is increased. Thus, the number of photolithography processes needed to form the TFT substrate is increased, thereby increasing production cost. For example, the TFT substrate having the structure shown in FIG. 1 and FIG. 3 of the Japanese Patent Application Laid-Open No. 2001-056474 is manufactured through six photolithography processes. For the TFT substrate of the conventional TN (Twisted Nematic) type, a manufacturing method with three photolithography processes is proposed as described above, and it is a big challenge in manufacturing the TFT substrate of the FFS type to reduce the number of photolithography processes (for example, Japanese Patent Application Laid-Open No. 2001-311965).
In order to solve the object, in Japanese Patent Application Laid-Open No. 2001-235763 and No. 2009-157366, methods are proposed in which the number of photolithography processes in manufacturing the TFT substrate of the FFS type is reduced to four to five. However, the number is still larger than that of the photolithography processes in manufacturing the TFT substrate of the TN type; thus, the production cost cannot be prevented from increasing.